(1) Field of the Invention
This invention relates to and is applicable to the desalination of seawater, brackish waters, industrial wastewater, municipal wastewater and, in general salty or saline water, or mining wastewater, by applying vertical tube evaporation or distillation.
(2) Description of the Prior Art
Known prior art related to this specification is described in the above U.S. Pat. No. 5,968,312 and in my U.S. Pat. No. 5,156,706 dated Oct. 20, 1992.
The U.S. Pat. No. 5,968,312 is about feed flow control and its distribution into a multiplicity of parallel vertical flash-down channels and into the tubes of a multi-effect series of evaporators arranged in a vertical array of such effects for seawater desalination or liquid evaporation, and by utilizing steam under pressure at an elevated temperature. Feed flow control and its distribution are provided for with adjustable orifice plates mounted horizontally and spaced above the inlet ends of a multiplicity of tubes in each of the tube bundles. Similar orifice plates are also applied therein to control flash-down of distillates, effect to effect to lower temperatures or pressure. The CIP application also references a Preliminary Design Report Number 1084 issued by the Metropolitan Water District of Southern California about a 30-effect vertical tube evaporator design which does not apply adjustable orifice plates or adjustable orifices but mentions fixed orifices drilled through distribution feed plates mounted above the inlet ends of vertical evaporator tube bundles. Each of those effect levels as well as the vertical tube bundles of the prior art designs operate at different steam and brine temperatures, i.e. by multi-effect mode, and at different Delta-T's that vary by typically about 3 to 5.degree. F. effect to effect. The reason for using such low Delta-T's per effect is to economize the high cost of the steam used at 233.degree. F. or 22 psi. However, low Delta-T's impose high capital costs per effect as well as an increase in the product water cost. The present invention eliminates the high cost of steam under pressure which typically contributes about 40% of the cost of water obtained by multi-effect evaporation. The present invention also provides cost savings based on the effective use of waste steam under a vacuum which overcomes the small Delta-T limitations imposed by high cost steam which increases the heat transfer surfaces needed and its associated higher capital cost; and this invention also reduces the complexity of those facilities and the cost of pure water produced by desalination. The present invention is based on a new path of steam flow by using large, stacked tube bundles, and also of vapor flow out of such bundles. In addition, it provides means for increasing and improving evaporator design options, especially the need to minimize or to control parasitic pressure losses in steam and vapor flow channels for and inside evaporators.
In U.S. Pat. No. 5,156,706 of 1992, single-effect waste-heat evaporators are disclosed having conventional tube bundles that are individually enclosed in shells, not stacked or staged, and the feed is recycled through such single tube bundles with a pump and is not cascaded from one tube bundle to another below it which distinguishes it from this case. This reference is also about the use of a monomolecular dispersant additive to control fooling and scaling of heat transfer surfaces during brine concentration with evaporators and about maintaining online operation for prolonged periods of time. It was aimed at the wastewater produced in power plants, typically at 50,000 to 500,000 gallons per day or GPD rates. The present target is what to do about waste heat, and the problem is that conventional evaporators have too small a capacity to handle it and cannot be scaled up by making their tube bundles taller or fatter, because both would reduce their efficiency unacceptably.
The present invention is direct to large capacity units, and to the control of steam-side and brine-side pressure drop to improve the thermal efficiency and to reduce the capital cost of desalination with large evaporation plants to a level lower than that obtainable by any other desalination procedure; and it is also distinguished from those procedures.
Waste heat for evaporation is available at a low temperature, in the range of about 100 to 120.degree. F. as turbine reflect steam under a vacuum, and this heat is usually rejected to a coolant passed through a condenser maintained under vacuum. This type of steam has no $-value on the bams that no further power can be generated with it in the turbine; it flows as waste in thousands of tons per hour in large power plants. Such heat sources may now become useful due this invention of an evaporator which can handle and use large rates of flow of waste steam, has a highly enhanced efficiency of heat transfer as described below, and is capable of being a substitute for the conventional power plant condenser. Since this type of heat or waste heat has essentially no $-value, the most economical option of using it for evaporation would be to apply it in a single-effect mode which minimizes the capital cost of the evaporator by applying the entire temperature difference (Delta-T, or D-T) available between the steam and the heat sink or coolant in a single-effect mode. Such an evaporator is distinctly differentiated from both the conventional vapor compression type of evaporator which usually operates at high temperatures (typically at about 220.degree. F.) as single-effect units with their vapor compressors being driven by electric motors, and also from the multi-effect evaporators which typically operate within the temperature range of about 220.degree. F. and 120.degree. F. In the latter, a step-wise series of effect temperatures are applied, to provide the beat or a reasonable economy based on the combination of the capital cost of the facility and the cost of the steam. The present invention is about a novel modular type of single-effect evaporator having a large unit capacity ranging from about 1 million gallons per day or MGD to 20 MGD, and wherein the concerns of control of steam and vapor pressure losses, and their efficient condensation, are of primary significance, especially when operating in single-effect mode at a low temperature but with the advantage of a relatively high Delta-T, this generates relatively higher steam and vapor mass flows due to this high Delta-T and the low temperature of operation in comparison to the above vapor compression and multi-effect units. Such an evaporator can either serve as the sole condenser for, or as one of several condensers for the steam being rejected from a turbine at low pressure or under vacuum. This type of evaporator-condenser can now be used to desalinate seawater at rates of about 20 MGD of distillate per 1000 MW steam turbine at coastal power plants, other wastewaters or brackish water sources may be desalinated or renovated for industrial or other uses at inland power plant locations, with this procedure.